The present invention relates to an automotive headlight which is used to secure the visual range in fog, rain, and snow.
Usually, this kind of lighting apparatus has such a structure that the filament (2) of the lamp bulb (1) is placed horizontally at the focus of the reflector (3) in the form of a paraboloid of revolution and the diffuser lens (4) is placed in the horizontal direction at the front of the reflector, as shown in FIGS. 1 and 2.
In such a structure, a multiplicity of points (p.sub.1)-(p.sub.8) are assumed on the reflector (3). They are (p.sub.1) at lower part, (p.sub.2) at diagonally left lower part, (p.sub.3) at left part, (p.sub.4) at diagonally left upper part, (p.sub.5) at upper part, (p.sub.6) at diagonally right upper part, (p.sub.7) at right part, and (p.sub.8) at diagonally right lower part. The respective independent light distribution patterns of the filament (2) at these points (p.sub.1)-(p.sub.8) are as shown in FIG. 1 when the lens (4) is removed. In other words, if the light reflected at each point (p.sub.1)-(p.sub.8) on the reflector (3) is projected on a vertical screen at a certain distance, with the front lens (4) removed, the shape of the filament (2) will be seen on the screen. Usually, these light distribution patterns (P.sub.1)-(P.sub.8) overlap each other on the screen. This state is shown in FIG. 3. The square corner part (5) of each pattern (P.sub.1)-(P.sub.8) is due to radiation of the leg (2') of the filament (2).
In FIG. 1, the samples of the light distribution patterns (P.sub.1)-(P.sub.8) have been extracted from the eight points (p.sub.1)-(p.sub.8) on the reflector (3). In actuality, the reflector (3) is an aggregate of an infinite number of points, and consequently the overlapped light distribution patterns are much more complex than FIG. 3. However, samples extracted from eight points are considered to represent the same light distribution patterns extracted from a multiplicity of points in view of the fact that the reflector (3) is a paraboloid of revolution and the sample points are equally spaced at the same radii from the focus, or the position of the filament.
The overlapped light distribution patterns (P.sub.1)-(P.sub.8) as shown in FIG. 3 become as shown in FIG. 4 when diffused in a long, narrow strip in the horizontal direction by the front lens (4). The light distribution pattern (6) as shown in FIG. 4 has a disadvantage that the upper part (6.sub.1) to (6.sub.2) is dim and flat. When a headlight (7) having the light distribution pattern (6) is mounted on an automobile, the beam of the light is separated into two parts as shown in FIG. 5. Thus, if such a headlight (7) is mounted in such a manner that the upper edge (6.sub.1) reaches the required distance (x), then the farthest edge (6.sub.2) of the light flux of the required illumination reaches only the distance (y), resulting in a shortage of illumination. If the headlight (7) is positioned upward in order to get necessary illumination, then the headlight will cause dazzle to the driver in a car running in the opposite direction. This phenomenon results from the fact that the upper edges (6.sub.1)(6.sub.2) do not coincide with each other and a dim part is formed at the overlapped light distribution patterns (P.sub.1)-(P.sub.8) as shown in FIG. 3. This phenomenon is enhanced by the fact that the filament (2) of the lamp bulb (1) does not necessarily coincide with the focus of the reflector (3) or the filament (2) is not accurately horizontal.
One way of obviating such an undesirable phenomenon is to lower a little the diagonal light distribution patterns (P.sub.2) (P.sub.4) (P.sub.6) (P.sub.8) which project beyond a certain line in FIG. 3. In order to accomplish this, a part of the front lens (4) through which these light distribution patterns pass should be formed into a refracting part which becomes thicker downward as shown in FIG. 7. Then, the above-mentioned light distribution patterns (P.sub.2) (P.sub.4) (P.sub.6) (P.sub.8) would be lowered a little and all the upper edges would coincide with each other. However, the lens cutting as mentioned above could not accomplish the expected objective, because the lens cutting as shown in FIG. 7 gives rise to upward diffused light at the stepped part (8). This upward diffused light is extremely harmful for a small headlight about 15 cm in diameter which should illuminate the road several tens of meters ahead with sufficient illumination to permit the driver to confirm any obstacles, and yet should not cause dazzle to the driver in a car running in the opposite direction.